Light olefins such as ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide. The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate-to-olefin (OTO) reaction process. One particularly preferred OTO process is a methanol-to-olefins (MTO) reaction process, wherein methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve catalyst.
Typically, the product stream from an MTO reactor is initially directed to a quench unit or tower for product quenching. In the quenching unit, the product stream contacts a quenching medium, usually water, under conditions effective to separate the product stream into a light product fraction and a heavy product fraction. The compounds in the product stream that are gaseous under the quenching conditions are separated therefrom as the light product fraction. The light product fraction typically contains light olefins, dimethyl ether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock. The light product fraction is compressed and directed to olefin product recovery and purification. The compounds in the product stream that are liquid under quenching conditions, are separated therefrom as the heavy product fraction. The heavy product fraction contains byproduct water, a portion of the unreacted oxygenate feedstock (except those oxygenates that are gases under quenching conditions), a small portion of the oxygenate conversion byproducts, particularly heavy hydrocarbons (C5+), and usually the bulk of the quench medium. The heavy product fraction may be processed to separate one or more of the heavy components contained therein. Exemplary non-limiting MTO separation systems are described in U.S. Pat. No. 6,121,504 and U.S. Pat. No. 6,482,998, the entireties of which are incorporated herein by reference and in U.S. patent application Ser. No. 10/383,204, filed Mar. 6, 2003, and U.S. patent application Ser. No. 10/292,232, filed Nov. 12, 2002, the entireties of which are also incorporated herein by reference.
Various byproducts are produced in the MTO reaction process. These byproducts may include organic or inorganic acids in the C1 to C6 range. These acids exit the MTO reactor in an MTO reaction effluent, which also includes the desirable light olefins formed in the MTO reaction process. A significant amount of carbon dioxide, which forms carbonic acid when dissolved in water, may also be present in the reaction effluent. These acidic components are usually divided between the light product fraction and the heavy product fraction. As a result, both the light and heavy product fractions are considerably acidic.
It has now been discovered that the acidity in the light and heavy fractions can cause localized corrosion in various regions of an MTO effluent processing system. Corrosion can reduce the thickness of pipe walls, ultimately leading to pipe weakening and failure, particularly at high pressures. Thus, the need exists for reducing corrosion in an MTO effluent processing system.